Industrial slings used in rigging or to lift, load, tow and/or move heavy loads are well-known in the art. At one time, industrial slings were made exclusively of wire-rope or chains. During the past twenty-five years, these industrial slings made of metal have seen improvements in flexibility and strength. However, despite the improvements, metal wire-rope slings still do not have the flexibility of non-metal (or synthetic) slings and have been largely replaced by non-metal slings.
Non-metal industrial slings can be made of natural or synthetic materials (especially those made of standard or high tenacity core yarns). Non-metal slings made of synthetic materials are usually called synthetic slings.
There are a number of methods of manufacturing non-metal slings, but the most efficient methods devise a way to form the load-bearing core into a substantially “endless” loop in which the ends of the cover are sewn together forming substantially a ring (and is usually referred to as an endless cover). Such non-metal or synthetic slings have the general shape of a ring and are called “roundslings.”
Thousands of roundslings are being used on a daily basis in a broad variety of heavy load-lifting applications which range from ordinary construction, plant and equipment operations, to ship building (e.g., oil rigs), nuclear power plants and the like. The lifting core fibers of such roundslings may be derived from natural or synthetic materials, such as polyester, polyethylene, nylon, and the like.
An advantage of synthetic slings is that they have a very high load-lifting performance (i.e., a high strength-to-weight ratio) which results in lighter, more flexible and even stronger slings than the heavier, relatively inflexible metal slings.
Non-metal industrial slings are comprised of a load-bearing core inside an elongated, tubular cover. The core bears the entire weight of the load to be lifted while the cover's sole function is to protect the core from physical damage and environmental exposure.
The cover protects the entire length of the core from damage. The cover not only protects the core from direct physical damage, such as sharp edges from the load and other objects that may come in contact with the sling, but also protects the core from molecular damage (e.g., chemicals/acids, ultraviolet degradation caused by sunlight, environmental pollutants, excessive heat under working conditions, etc.).
The load-bearing core of a synthetic sling is made of a number of core yarns (sometimes called core strands). Each core yarn is made of a plurality of threads.
Sling manufacturers of prior art synthetic slings wind the core yarns into an endless loop in which each run or loop of the core yarn is substantially parallel to every other loop (this may be referred to as load-bearing core having core yarns laid straight). The winding is usually performed on a machine having a motor-driven roller and a free-rolling roller set a specific distance away from the motor-driven roller. During the manufacturing process, the cover is “bunched” together in accordion-like fashion, and the core yarns are run straight through the cover. The distance between rollers is determined by the desired length of the sling to be made.
Before a sling manufacturer begins making a sling, it must know how much weight the sling needs to support (i.e., the rated load), determine how much force or weight each individual core yarn can support, then calculate how many loops are needed to make the load-bearing core for the rated load. Many factors can impact these calculations including the type of material selected for the core yarns, the diameter of the yarns, the diameter of the threads used to make the core yarns, etc.
Non-metal slings are not without their own unique problems. It has been discovered that this method of manufacturing results in loops of core yarns that are of slightly different lengths. Therefore, when a prior art non-metal sling is placed under load, the force of the load is borne by the shortest loops of core yarn. In other words, a load-bearing core is designed to have “X” number of core yarns to be able to life the rated load, but only a fraction of the “X” number of core yarns bear the weight of the load because of the differences in lengths of each loop. This prior art configuration can result in the shortest loops being damaged because they are overloaded until they eventually break. When the shortest loops of core yarns break, the next shortest loops of core yarns support the load until they too are damaged and eventually break, and so on until the synthetic sling suffers a catastrophic failure.
In order to prevent a catastrophic failure, synthetic slings must be constantly inspected and/or tested to ensure that they continue to meet the load they are rated to support. If damage to the load-bearing core is discovered, the sling is removed from service and destroyed.